U.S. patent application number 13/516945 was filed with the patent office on 2012-10-18 for operation method for mechanically stirring chrome-containing molten iron.
This patent application is currently assigned to NISSHIN STEEL CO., LTD.. Invention is credited to Masakazu Mori, Masayuki Sugiura, Takahiro Yoshino.
Application Number | 20120260773 13/516945 |
Document ID | / |
Family ID | 44305393 |
Filed Date | 2012-10-18 |
United States Patent
Application |
20120260773 |
Kind Code |
A1 |
Sugiura; Masayuki ; et
al. |
October 18, 2012 |
OPERATION METHOD FOR MECHANICALLY STIRRING CHROME-CONTAINING MOLTEN
IRON
Abstract
A method for stirring chrome-containing molten iron comprises
mechanically stirring chrome-containing molten iron contained in a
refining vessel by the use of an impeller having a rotation axis in
the vertical direction where the refining vessel is such that the
horizontal cross section of the inner wall thereof is circular
around the central axis of the vessel in the vertical direction and
the impeller, as integrated with the axial rod covered with a
refractory, rotates around the central axis of the axial rod, as
the rotation axis thereof. The stirring mode is regularly or
irregularly switched between a concentric stirring of the molten
iron in a state where the rotation axis of the impeller is centered
in the central axis of the vessel and an eccentric stirring of the
molten iron in a state where the rotation axis of the impeller is
decentered from the central axis of the vessel.
Inventors: |
Sugiura; Masayuki;
(Yamaguchi, JP) ; Mori; Masakazu; (Yamaguchi,
JP) ; Yoshino; Takahiro; (Yamaguchi, JP) |
Assignee: |
NISSHIN STEEL CO., LTD.
Tokyo
JP
|
Family ID: |
44305393 |
Appl. No.: |
13/516945 |
Filed: |
December 8, 2010 |
PCT Filed: |
December 8, 2010 |
PCT NO: |
PCT/JP2010/072051 |
371 Date: |
June 18, 2012 |
Current U.S.
Class: |
75/583 |
Current CPC
Class: |
C21C 1/06 20130101; F27D
27/00 20130101; C21C 1/02 20130101; C21C 7/064 20130101; C21C 1/04
20130101 |
Class at
Publication: |
75/583 |
International
Class: |
C21C 7/00 20060101
C21C007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 7, 2010 |
JP |
2010-002408 |
Claims
1. An operation method for mechanically stirring chrome-containing
molten iron, which comprises a refining process of mechanically
stirring chrome-containing molten iron contained in a refining
vessel by the use of an impeller having a rotation axis in the
vertical direction where the refining vessel is such that the
horizontal cross section of the inner wall thereof is circular
around the central axis of the vessel in the vertical direction and
the impeller, as integrated with the axial rod covered with a
refractory, rotates around the central axis of the axial rod, as
the rotation axis thereof, wherein: the stirring mode is regularly
or irregularly switched, as selected for each stirring charge,
between "concentric stirring mode" of stirring the molten iron in a
state where the rotation axis of the impeller is centered in the
central axis of the vessel and "eccentric stirring mode" of
stirring the molten iron in a state where the rotation axis of the
impeller is decentered from the central axis of the vessel.
2. The operation method for mechanically stirring chrome-containing
molten iron as claimed in claim 1, wherein the concentric stirring
mode and the eccentric stirring mode are alternately switched at
every one stirring charge.
3. The operation method for mechanically stirring chrome-containing
molten iron as claimed in claim 1, wherein the chrome-containing
molten iron is molten pig-iron or molten steel to be formed into
stainless steel in the subsequent step of another refining and
casting.
4. The operation method for mechanically stirring chrome-containing
molten iron as claimed in claim 1, wherein the chrome-containing
molten iron is molten pig-iron having a Cr content of from 8 to 35%
by mass.
5. The operation method for mechanically stirring chrome-containing
molten iron as claimed in claim 1, wherein, in the eccentric
stirring mode, the rotation axis of the impeller is decentered from
the central axis of the vessel within a range of from 0.20 D to
0.45 D where D (mm) means the initial axial rod diameter that
indicates the refractory diameter in the initial state of the axial
rod part sinking below the fluid level of the molten material
before the start of the rotation.
6. The operation method for mechanically stirring chrome-containing
molten iron as claimed in claim 1, wherein the initial axial rod
diameter D is from 10 to 30% of D.sub.0 where D.sub.0 (mm) means
the inner diameter of the refining vessel at the position of the
height of the mean fluid level of the molten matter being stirred.
Description
TECHNICAL FIELD
[0001] The present invention relates to an operation method of
reducing erosion of an axial rod part rotating integrally with
mixing blades (impeller) in a refining process of mechanically
stirring chrome-containing molten iron (molten pig-iron or molten
steel) with an impeller.
BACKGROUND ART
[0002] A refining process of mechanically stirring molten iron with
an impeller has heretofore been applied mainly to desulfurization
of blast furnace-derived molten pig-iron (for example, Patent
References 1 to 4). In that regard, for enhancing the stirring
efficiency, proposed is a stirring method where the rotation axis
of the impeller is kept decentered from the central axis of the
refining vessel (Patent Reference 3). According to the method, it
is said that the revolutions per minute (r.P.m) could be reduced in
a case of obtaining a predetermined desulfurization efficiency, and
the life of the impeller could be prolonged.
[0003] On the other hand, in the production of molten stainless
steel, a process of obtaining molten pig-iron or molten steel by
the use of an electric furnace is the mainstream. In that case,
CaF.sub.2 (fluorite) may be incorporated in the slag in the
electric furnace, or CaF.sub.2 may be incorporated in the slag
during the decarburizing stage, whereby the desulfurization can be
attained relatively efficiently, and accordingly, a step of
mechanically stirring molten pig-iron or molten steel is not
specifically needed.
[0004] Recently, however, in use of steel slag as a ground or
roadbed material, the content of the fluorine ingredient therein
has become restricted, and therefore use of CaF.sub.2-free slag has
increased. In that case, the desulfurization capability of slag
lowers, and therefore, in case where an ultra-low S molten
stainless steel having an S content of, for example, at most 0.005%
by mass is produced, it has become necessary to apply separate
desulfurization treatment to the electric furnace molten pig-iron
or steel for the purpose of reducing the desulfurization load in
the already-existing steel-making process
[0005] Regarding the desulfurization treatment, it has been
confirmed that the same mechanical stirring method as that for
blast furnace molten pig-iron is effective also for
chrome-containing molten pig-iron or steel for stainless steel. For
example, in case where CaO is used as the desulfurizing agent and
when chrome-containing molten pig-iron or steel is mechanically
stirred along with the desulfurizing agent (CaO-based slag), then
the desulfurization reaction of the following formula (1) goes on.
The generated oxygen reacts with the deoxidizing ingredient (for
example, Si) in the molten iron, according to the following formula
(2):
(CaO)+[S]=(CaS)+[O] (1)
[Si]+2[O]=(SiO.sub.2) (2)
CITATION LIST
Patent References
[0006] Patent Reference 1: JP-A 2004-248975
[0007] Patent Reference 2: JP-A 2001-248976
[0008] Patent Reference 3: JP-A 2001-262212
[0009] Patent Reference 4: JP-A 2003-166010
SUMMARY OF THE INVENTION
Problems that the Invention is to Solve
[0010] As described above, in a case of mechanically stirring
molten pig-iron or molten steel, when the stirring is attained in a
state where the rotation axis of the impeller is decentered from
the central axis of the refining vessel (eccentric stirring), then
the stirring efficiency increases and therefore the revolutions per
minute can be reduced for attaining the same desulfurization
effect. However, in the present inventors' investigations, there
occurred a problem in that, in the case of chrome-containing molten
pig-iron or steel as differing from the case of blast furnace
molten pig-iron, the refractory part of the axial rod that rotates
integrally with the impeller is extremely readily eroded or melted
during the operation of eccentric stirring (see FIG. 5 to be
mentioned below). Consequently, even though the life of the
impeller itself could be prolonged, the life of the axial rod part
early comes to the end thereof, and therefore the change-out rate
of the "rotor" composed of the impeller and the axial rod integral
with each other is shortened.
[0011] In consideration of the situation as above, the present
invention is to provide an operation method for noticeably
prolonging the life of the "rotor" composed of an impeller and an
axial rod integral with each other in mechanical stirring of
chrome-containing molten pig-iron or steel.
Means for Solving the Problems
[0012] As a result of detailed investigations, the present
inventors have found that, in mechanical stirring of
chrome-containing molten pig-iron or steel, there exists a
noticeable difference between the case of stirring in a state where
the rotation axis of an impeller is centered in the central axis of
a refining vessel(concentric stirring mode) and the case of
stirring in a state where the former is decentered from the latter
(eccentric stirring mode), in the material loss of the axial rod
part that rotates integrally with the impeller. Specifically, in
the case of the eccentric stirring mode, the material loss of the
axial rod is extremely large, as described above. As opposed to
this, in the case of the concentric stirring mode, scattered
matters of slag and molten pig-iron or molten steel may readily
adhere to the axial rod. Moreover, the materials adhered are hard
and could not peel away with ease but have an effect of firmly
protecting the refractory part of the axial rod. In other words,
during stirring operation in the concentric stirring mode, a hard
protective layer of the adhesion materials is naturally formed on
the surface of the refractory axial rod, and therefore in this
description, this phenomenon may be referred to as
"self-repairing".
[0013] The refractory axial rod eroded in stirring in the eccentric
stirring mode could be self-repaired by changing the subsequent
stirring mode to the concentric stirring mode. Afterwards, by
repeating the eccentric stirring mode and the concentric stirring
mode, the amount of the slag and the scattering matters to be
adhered to the axial rod can be controlled, whereby consequently
the life of the refractory axial rod can be greatly prolonged. The
present invention has been completed on the basis of these
findings.
[0014] Specifically, according to the invention, there is provided
an operation method for mechanically stirring chrome-containing
molten iron, which comprises a refining process of mechanically
stirring chrome-containing molten iron contained in a refining
vessel by the use of an impeller having a rotation axis in the
vertical direction where the refining vessel is such that the
horizontal cross section of the inner wall thereof is circular
around the central axis of the vessel in the vertical direction and
the impeller, as integrated with the axial rod covered with a
refractory, rotates around the central axis of the axial rod, as
the rotation axis thereof, wherein:
[0015] the stirring mode is regularly or irregularly switched, as
selected for each stirring charge, between "concentric stirring
mode" of stirring the molten iron in a state where the rotation
axis of the impeller is centered in the central axis of the vessel
and "eccentric stirring mode" of stirring the molten iron in a
state where the rotation axis of the impeller is decentered from
the central axis of the vessel.
[0016] As one embodiment of regularly switching the mode,
preferably employed here is a method where the concentric stirring
mode and the eccentric stirring mode are alternately switched at
every one stirring charge.
[0017] As the chrome-containing molten iron, more effectively used
here is molten pig-iron or molten steel having a Cr content (at the
start of stirring of each stirring charge) of from 8 to 35% by
mass. One typical candidate is molten pig-iron or molten steel
which is to be formed into stainless steel by another subsequent
refining process and casting. "Stainless steel" as referred to
herein is defined as Number 3801 of JIS G0203:2009, and the steel
includes concretely austenitic steel types defined in Table 2 of
JIS G4305:2005, austenitic ferritic steel types defined in Table 3
thereof, ferritic steel types defined in Table 4 thereof,
martensitic steel types defined in Table 5 thereof, precipitation
hardened steel types defined in Table 6 thereof; and in addition to
these, other various types of developed steel not corresponding to
JIS could also be the objects of the invention. Especially
preferred objects are ultra-low S steel types (for example, having
an S content of at most 0.005% by mass) with the base of those
ingredient systems.
[0018] Especially preferably, in the eccentric stirring mode, the
rotation axis of the impeller is decentered from the central axis
of the vessel within a range of from 0.20 D to 0.45 D where D (mm)
means the initial axial rod diameter that indicates the refractory
diameter in the initial state of the axial rod part sinking below
the fluid level of the molten matter before the start of the
rotation. The initial axial rod diameter D may be within a range of
from 10 to 30% of D.sub.0 where D.sub.0 (mm) means the inner
diameter of the refining vessel at the position of the height of
the mean fluid level of the molten matter being stirred.
[0019] The "molten matter" as referred to herein means a substance
in a molten state in the refining vessel, concretely including
chrome-containing molten iron (molten pig-iron or molten steel),
and flux for refining and slag to be stirred along with it. The
"position of the height of the mean fluid level of the molten
matter being stirred" corresponds to the position of the height of
the mean fluid level of the molten matter on the assumption that
the stirring is stopped and the fluid level is kept static. In case
where the height of the mean fluid level fluctuates, for example,
in such a case that flux or the like is put into the system in the
course of stirring, the highest position is employed.
ADVANTAGE OF THE INVENTION
[0020] According to the invention, in mechanically stirring
chrome-containing molten iron (molten pig-iron or molten steel),
the period of time to exchange the rotor that comprises an impeller
integrated with the axial rod thereof can be greatly prolonged.
Accordingly, the invention contributes toward performance increase
and cost reduction in the step of promoting reaction by mechanical
stirring, such as desulfurization treatment or reduction and
recovery of chrome from the slag, in a process of refining
chrome-containing steel such as typically stainless steel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] [FIG. 1] This is a view schematically illustrating the shape
of a rotor in the initial state thereof.
[0022] [FIG. 2] This is a partial cross-sectional view
schematically showing the configuration of each part in a refining
vessel in which chrome-containing molten iron is mechanically
stirred in a concentric stirring mode.
[0023] [FIG. 3] This is a view schematically illustrating the
outward appearance of a rotor that is to be exchanged in continuous
mechanical stirring of chrome-containing molten iron in a
concentric stirring mode.
[0024] [FIG. 4] This is a partial cross-sectional view
schematically showing the configuration of each part in a refining
vessel in which chrome-containing molten iron is mechanically
stirred in an eccentric stirring mode.
[0025] [FIG. 5] This is a view schematically illustrating the
outward appearance of a rotor that is to be exchanged in continuous
mechanical stirring of chrome-containing molten iron in an
eccentric stirring mode.
[0026] [FIG. 6] This schematically illustrates the outward
appearance of a rotor that is considered to be still usable in a
case where mechanical stirring of chrome-containing molten iron is
continued while the stirring mode is switched alternately between a
concentric stirring mode and an eccentric stirring at every one
stirring charge.
MODE FOR CARRYING OUT THE INVENTION
[0027] FIG. 1 schematically illustrates the configuration of a
rotor to be applied to the mechanical stirring in the invention, in
the initial state thereof (before used). An impeller 2 is fitted to
the lowest part of axial core 1 formed of a steel material or the
like. Inside the impeller 2, in general, there exists a core
material (not shown) formed of a steel material, as connected with
the axial core 1, and the impeller 2 is constructed by covering the
core material serving as a base with a refractory. Around the axial
core 1, formed is a refractory layer 3 so as to protect the axial
core 1 formed of a steel material or the like from being directly
exposed to a molten material. An axial rod 10 is composed of the
axial core 1 and refractory layer 3 around it. The impeller 2 and
the axial rod 10 rotate integrally with each other. The integrated
structure is referred to as a rotor 20.
[0028] FIG. 2 schematically shows the configuration of each part in
a refining vessel in which chrome-containing molten iron is
mechanically stirred in a concentric stirring mode. This shows a
cross section of the vessel including central axis 40 thereof and
rotation axis 41, in which only rotor 20 is shown as the side view
thereof (the same shall apply to FIG. 4 to be mentioned below).
[0029] Refining vessel 30 to be used here is such that the
horizontal cross section of inner wall 33 thereof is circular
around central axis 40 of the vessel in the vertical direction. The
"horizontal cross section" is a cross section vertical to the
central axis 40 of the vessel standing in the vertical direction.
"Circular" accepts ordinary irregularities (deviation from perfect
circle) to occur in constructing inner wall 33 from a refractory.
The inner diameter of the refining vessel 30 may be uniform in the
height direction or may not be uniform. For example, a refining
vessel of which the inner diameter increases upward from the bottom
may be used here.
[0030] The rotor 20 is so designed that the upper part of the axial
rod 10 thereof is fixed to the rotary member that is rotated by the
driving force of a motor, and by changing the position of the
rotary member, the height position and the horizontal position of
the rotor 20 can be set at predetermined positions. In the
concentric stirring mode, rotation axis 41 and central axis 40 of
the vessel correspond to each other, and therefore, when the
stirring with the rotor 20 is started, then the eddy core 50 of the
fluid formed of chrome-containing molten iron 31 and flux and/or
slag 32 is formed at the center position of the refining vessel 30.
With that, the molten material level is low at the position of the
eddy core 50 and is high at around the peripheral part. In FIG. 2,
the molten material level fluctuation is overdrawn (the same shall
apply to FIG. 4 to be mentioned below). With the rotation, the
interface between chrome-containing molten iron 31 and flux and/or
slag 32 maybe complicated, but in FIG. 2, the interface is drawn in
a simplified manner (the same shall apply to FIG. 4 to be mentioned
below). The height position of the rotor 20 is so set that the top
of the impeller 2 could be lower than the molten material level of
the eddy core 50. The upper open mouth of the refining vessel 30 is
closed mostly with hood 34 except the area around the axial rod
10.
[0031] When the molten iron is stirred in a concentric stirring
mode, the adhesion material layer caused by slag, molten pig-iron
or molten steel is formed onto axial rod 10 in the part near the
molten material surface and in the part upper than the molten
material surface, during rotation of the axial rod 10. The adhering
amount of the adhesion material tends to be considerably large as
compared with that in stirring of blast furnace pig-iron. Moreover,
the adhesion material layer is hard. The present inventors analyzed
the adhesion material formed in stirring of chrome-containing
molten pig-iron or steel, and have found that the material contains
a chromium oxide ingredient. It is presumed that the specific
composition of the adhesion material would contribute toward
self-repairing of the eroded part of the refractory axial rod, as
described below.
[0032] FIG. 3 schematically illustrates the outward appearance of a
rotor after about 50 charges in continuous mechanical stirring of
chrome-containing molten pig-iron or steel in a concentric stirring
mode. The surface of the refractory layer 3 to constitute the axial
rod 10 is covered thickly with hard adhesion material 4. In that
condition, it is extremely difficult to remove the adhesion
material 4 with hammer or any other tool. In addition, when the
apparent diameter of the axial rod 10 increases more owing to the
adhesion material 4, then the amount of the slag or the molten
metal to scatter during rotation may increase more and the adhering
speed of the adhesion material 4 thereby increases more and more.
Consequently, in case where the mechanical stirring of
chrome-containing molten iron is attained only in a concentric
stirring mode, the rotor must be frequently exchanged.
[0033] FIG. 4 schematically shows the configuration of each part in
a refining vessel in which chrome-containing molten pig-iron or
steel is mechanically stirred in an eccentric stirring mode. The
rotor 20 rotates in the condition where the rotation axis 41
thereof is decentered from the central axis 40 of the vessel by the
eccentric degree .delta.. In this case, the eddy core 50 is shifted
to the opposite side to the rotation axis 41 relative to the
central axis 40 of the vessel. The degree of shifting of the eddy
core 50 from the center position of the vessel is nearly the same
as the eccentric degree .delta. Also in the eccentric stirring
mode, the height position of the rotor 20 is so set that the top of
the impeller 2 could be lower than the molten metal level of the
eddy core 50.
[0034] Also in the eccentric stirring mode, slag and molten metal
may scatter from the molten metal surface. However, though the
adhesion material layer caused by the scattering is formed
extremely easily in the concentric stirring mode, the adhesion
could extremely hardly occur to such part of the axial rod 10 that
is washed by the fluctuation in the molten material surface level
in the eccentric stirring mode. Moreover, it has been clarified
that the refractory layer 3 in that part is extremely easily
eroded.
[0035] FIG. 5 schematically illustrates the outward appearance of a
rotor after about 150 charges of continuous mechanical stirring of
chrome-containing molten pig-iron or steel in an eccentric stirring
mode. The adhesion material 4 could be seen on partial surface of
the refractory layer 3 that constitutes the axial rod 10, but the
refractory layer 3 of other part that is washed by the molten
material surface was greatly eroded or melted thereby giving an
eroded refractory part 5 that was thinned to have a smaller
diameter than the diameter of the initial refractory layer 3. When
the diameter of the eroded refractory part 5 approaches to the
diameter of the axial core 1, then further use of the rotor 20 must
be evaded and the rotor must be exchanged. The number of charges to
reach that state may vary depending on the condition, but in usual
operation, the life of the rotor falls between about 80 and 180
charges in many cases. Blast furnace molten pig-iron does not
almost bring about such a problem of remarkable erosion or melt
even when continuously stirred in an eccentric stirring mode.
Rather in such a case, the wear and tear of the impeller 2 is often
a determinative factor of the life of the rotor 20. The reason why
chrome-containing molten iron causes the above-mentioned severe
erosion is not always clarified as yet at least at present;
however, it may be considered that a large amount of Cr that is an
easily-oxidizable element is contained in molten pig-iron and
molten steel and would be a factor of facilitating the erosion of
refractory. In addition, another reason would be that the
temperature of the molten pig-iron or molten steel to be stirred is
relatively high.
[Operation Method of the Invention]
[0036] In the invention, while one rotor 20 is continuously used,
not exchanged during the term, the operation is switched regularly
or irregularly between a concentric stirring mode and an eccentric
stirring mode, as selected for every stirring charge. With the
charge stirred in an eccentric stirring mode, the erosion of the
axial rod 10 goes on as mentioned above. With the subsequent charge
stirred in a concentric stirring mode, the eroded part of the axial
rod 10 is coated with a hard adhesion material, thereby exhibiting
the above-mentioned "self-repairing" effect. In that manner,
frequently repeating the "erosion" in the eccentric stirring mode
and the "self-repairing" in the concentric stirring mode makes it
possible to control the adhering amount of the adhesion material to
the axial rod 10 whereby the erosion of the refractory layer 3 that
constitutes the axial rod 10 can be greatly reduced. The layer of
the adhesion material formed in the concentric stirring mode is
mostly melted away in the subsequent eccentric-mode stirring
charges, and accordingly, the state where the axial rod 10 is
covered with the excessive adhesion material 4 as shown in FIG. 3
could be thereby evaded.
[0037] As one embodiment where the concentric stirring mode and the
eccentric stirring mode are regularly selected for each stirring
charge, for example, there is mentioned an embodiment where the two
modes are alternately switched at every one charge. In addition,
other preferred embodiments may be determined for the prolongation
of the life of the rotor 20 based on previous experimental data and
past operation data in accordance with (i) the condition of the
apparatus, (ii) the composition of chrome-containing molten
pig-iron or steel to be stirred, the composition of slag, and the
temperature condition thereof, (iii) the stirring condition, etc.
For example, there may be mentioned an embodiment where a cycle of
"eccentric stirring mode.times.two times.fwdarw.concentric stirring
mode.times.one time" is repeated. Also employable here is a
"variable pattern" where the mode switching pattern is changed
depending on the rotor use frequency.
[0038] Regarding the method of irregularly selecting the two modes
for every stirring charge, there is mentioned a method that
comprises measuring the eroded amount of the refractory layer 3 or
the adhering amount of the adhesion material 4 after every one
charge or at regular charge intervals and then determining the
stirring mode for the subsequent charges before the next
inspection.
[0039] It is effective that the eccentric degree .delta. (the
distance between the central axis 40 of the vessel and the rotation
axis 41) in the eccentric stirring mode is set in accordance with
the diameter of the axial rod 10. The diameter of the axial rod 10
in this case may be based on the diameter thereof of the rotor 20
before use in the first charge (the diameter in the unused state).
In this description, that diameter is referred to as "initial axial
rod diameter" and is represented by a symbol D. The initial axial
rod diameter D (mm) is the refractory diameter in the initial state
of the axial rod part sinking below the fluid level of a molten
material before the start of the rotation (or that is, in case
where the molten surface level is equivalent in the vessel). In
case where the diameter of the axial rod part varies in different
sites (for example, in case where the outer diameter of axial rod
10 varies in the height direction), the diameter of the thinnest
part of the axial rod part may be taken as the initial axial rod
diameter D. Using the rotor 20 is especially effective in which the
initial axial rod diameter D is from 15 to 30% of the inner
diameter D.sub.0 of the refining vessel (as mentioned above).
[0040] As a result of various investigations, the eccentric degree
.delta. is effectively at least 0.20 D in an eccentric stirring
mode. When the eccentric degree .delta. is smaller than the above,
then the predominance with occurring "erosion of the refractory
layer 3" and "adhering of adhesion material 4" may be unstable, and
it may be often difficult to stably realize the stirring condition
in which the erosion is predominant. The upper limit of the
eccentric degree .delta. may be physically restricted by the size
of impeller 2 and refining vessel 30 and is therefore unnecessary
to be specifically defined. However, larger .delta. is not always
effective but too large .delta. may be a cause of cost increase. In
addition, when .delta. is too large, then the impeller during
rotation may vibrate too much and may cause device failure. In
general, the eccentric degree .delta. falling within a range of
from 0.20 D to 0.45 D could produce a good result. The degree may
be controlled to fall within a range of from 0.20 D to 0.40 D, or
within a range of from 0.20 D to 0.35 D.
[0041] On the other hand, in the concentric stirring mode, the
rotation axis 41 may be misaligned somewhat from the predetermined
position owing to inevitable equipment-related reasons. As a result
of various investigations, the degree of misalignment is acceptable
up to 0.10 D. When the degree of misalignment is more than 0.10 D,
then the predominance with occuring "erosion of the refractory
layer 3" and "adhering of adhesion material 4" may be unstable, and
it may be often difficult to stably realize the stirring condition
in which the adhering is predominant. More preferably, the degree
of misalignment is suppressed to be at most 0.05 D.
[0042] The size of the refining vessel is not specifically defined.
For example, the invention is applicable to the vessel of which the
above-mentioned inner diameter D.sub.0 is from 1000 to 4500 mm or
so.
[0043] FIG. 6 schematically illustrates the outward appearance of a
rotor after about 150 charges of continuous mechanical stirring of
chrome-containing molten pig-iron or steel in a concentric stirring
mode and an eccentric stirring mode alternately switched at every
one charge. The condition of the rotor in this case is the same as
that in the above-mentioned FIG. 4 except that the two modes are
switched; and in this case, owing to the above-mentioned
"self-repairing effect", the erosion loss of the refractory at the
eroded part 5 could reduce and the rotor can be further used still
continuously.
EXAMPLES
[0044] Electric furnace molten pig-iron in a production of molten
stainless steel was desulfurized according to a method of
mechanically stirring it with a rotor. In the case, one rotor was
continuously used until its life (when the rotor came to be
exchanged), and on the basis of the pass counts (number of
processed stirring charges) therewith, the relative merits of the
mechanical stirring operation with the rotor (Examples shown in
Table 1) were evaluated.
[0045] As the refining vessel, used here was a ladle having a
cylindrical inner wall and having an inner diameter D.sub.0 of 2760
mm.
[0046] As the rotor, used here is one having the initial shape
shown in FIG. 1. The diameter of the refractory layer 3 is uniform
in the height direction. Accordingly, the dimension expressed as d
in FIG. 1 corresponds to the initial axial rod diameter D. The
value D in each Example is shown in Table 1. The dimension of the
impeller 2 is w=1200 mm and h=700 mm in FIG. 1; and the blade
thickness a is nearly the same as the initial axial rod diameter D.
The dipping depth of the rotor is, based on the molten material
level in a state where the rotor is kept static, was so controlled
that the depth from the molten material surface to the top of the
impeller could be 500 mm. The stirring time in one charge was 600
seconds, and the revolution number of the rotor was within a range
of from 80 to 120 r.p.m.
[0047] The amount of chrome-containing molten pig-iron to be
stirred in one charge is about 80 tons. Regarding the type of the
pig-iron treated here, Fe--Cr--Ni-based molten pig-iron for
austenitic stainless steel accounted for from about 40 to 60% of
all the stirring charges until the life of the rotor, and
Fe--Cr-based molted pig-iron for ferritic stainless steel accounted
for the remaining stirring charges. The temperature of the
chrome-containing molten pig-iron at the start of stirring was
within a range of from 1390 to 1450.degree. C.
[0048] After every charge, the "diameter of the axial rod part" and
the "erosion loss of the impeller" were checked, and when any of
either measured up to the standard, the life of the rotor was
considered to have come an end. The outer diameter standard of the
axial rod part was at the time when the diameter of the most-eroded
part became more than [initial axial rod diameter D--100 mm], or
when the apparent outer diameter of the axial rod became thick
owing to adhering the adhesion material thereto and further use of
the rotor would cause some trouble owing to the increase in the
scattering amount of slag or molten pig-iron or owing to unstable
rotation of the rotor. The erosion loss standard of the impeller
was at the time when the intended desulfurization of chromium
reduction recovery could not be attained within a predetermined
period of time (600 seconds) if the revolution number is not
increased up to 130 r.p.m. or more.
[0049] The operation condition and the result in each Example are
shown in Table 1. In this, in the Example where the expression
"regular" is given to the column of mode switching pattern, the
concentric stirring mode and the eccentric stirring mode were
alternately switched at every stirring charge. In the Example where
the expression "irregular" is give thereto, the erosion loss of the
refractory layer 3 or the adhered amount of adhesion material 4 was
checked after every charge, and in case where self-repairing by the
adhesion was considered to be necessary in the next charge, the
concentric stirring mode was selected, and in the other cases, the
eccentric stirring mode was selected, and in that manner, the two
modes were suitably switched. However, the same stirring mode must
not be continued 3 times or more. In the Example where
"CaO--Al.sub.2O.sub.3" is given to the column of slag, all charges
are for desulfurization.
TABLE-US-00001 TABLE 1 Initial Eccentric Axial Rod Mode Degree
.delta. Rotor Life Rotor Diameter D D/D.sub.0 .times. Switching in
Eccentric (number of Cause of Example No. No. (mm) 100 (%) Stirring
Mode Pattern Stirring Mode Slag *1 charges) Rotor Life Comparative
1 550 19.9 eccentric mode alone -- 0.30D CaO--Al.sub.2O.sub.3 143
axial rod Example 1 erosion Comparative 2 580 21.0 eccentric mode
alone -- 0.26D CaO--Al.sub.2O.sub.3 174 axial rod Example 2 erosion
Comparative 3 600 21.7 eccentric mode alone -- 0.25D
CaO--Al.sub.2O.sub.3 99 axial rod Example 3 erosion Comparative 4
580 21.0 concentric mode alone -- -- CaO--Al.sub.2O.sub.3 50 axial
rod Example 4 thickening Example 1 5 500 18.1 concentric/eccentric
regular 0.30D CaO--Al.sub.2O.sub.3 281 axial rod combined mode
erosion Example 2 6 550 19.9 concentric/eccentric regular 0.30D
CaO--Al.sub.2O.sub.3 318 axial rod combined mode erosion Example 3
7 580 21.0 concentric/eccentric regular 0.26D CaO--Al.sub.2O.sub.3
204 axial rod combined mode erosion Example 4 8 580 21.0
concentric/eccentric regular 0.20D to CaO--Al.sub.2O.sub.3 298
axial rod combined mode 0.45D erosion Example 5 9 600 21.7
concentric/eccentric regular 0.25D CaO--Al.sub.2O.sub.3 324 axial
rod combined mode erosion Example 6 10 650 23.6
concentric/eccentric regular 0.20D CaO--Al.sub.2O.sub.3 266 axial
rod combined mode erosion Example 7 11 580 21.0
concentric/eccentric irregular 0.26D CaO--Al.sub.2O.sub.3 312 axial
rod combined mode erosion *1 Type of slag in stirring treatment of
every charge
[0050] As seen from Table 1, the life of the rotor was extremely
prolonged in Examples where the two modes were suitably switched,
as compared with that in Comparative Examples where all the charges
were processed in the eccentric stirring mode alone or in the
concentric stirring mode alone.
DESCRIPTION OF REFERENCE NUMERALS
[0051] 1 Axial Core [0052] 2 Impeller [0053] 3 Refractory Layer
[0054] 4 Adhesion Material [0055] 5 Refractory Eroded Part [0056]
10 Axial Rod [0057] 20 Rotor [0058] 30 Refining vessel [0059] 31
Chrome-Containing Molten Iron [0060] 32 Flux and/or Slag [0061] 33
Inner Wall [0062] 34 Hood [0063] 40 Central Axis of Vessel [0064]
41 Rotation Axis [0065] 50 Eddy Core
* * * * *